The carbonyl stretch C=O of saturated aliphatic aldehydes appears from 1740-1720 cm-1. As in ketones, if the carbons adjacent to the aldehyde group are unsaturated, this vibration is shifted to lower wavenumbers, 1710-1685 cm-1. See also:
Another useful diagnostic band for aldehydes is the O=CH stretch. This band generally appears as one or two bands of moderate intensity in the region 2830-2695 cm-1. Since the band near 2830 cm-1 is usually indistinguishable from other CH stretching vibration bands (recall that the CH stretches of alkanes appear from 3000-2850 cm-1), the presence of a moderate band near 2720 cm-1 is more likely to be helpful in determining whether or not a compound is an aldehyde.
If you suspect a compound to be an aldehyde, always look for a peak around 2720 cm-1; it often appears as a shoulder-type peak just to the right of the alkyl CH stretches.
The spectra of benzaldehyde and butyraldehyde are shown below. Note that the O=C stretch of the alpha, beta-unsaturated compound -- benzaldehyde -- is at a lower wavenumber than that of the saturated butyraldehyde.
Note the O=CH stretches in both aldehydes in the region 2830-2695 cm-1, especially the shoulder peak at 2725 cm-1 in butyraldehyde and 2745 cm-1 in benzaldehyde.